1. Field of the Invention
The invention relates to a method for producing a polished semiconductor wafer, the semiconductor wafer being cut from a crystal and subjected to a series of processing steps.
2. Background Art
After having cut semiconductor wafers from a crystal, for example a silicon crystal, the production of polished semiconductor wafers requires a series of processing steps which can be broadly divided into two groups according to their purpose: shaping process steps (wafering steps) and smoothing process steps. The shaping process steps are intended in particular to give the semiconductor wafers a shape which is distinguished by a rounded edge and plane-parallel opposite sides. In the further course of the method at least one of the sides, usually the front side, is brought into a polished state in which it then only has minor roughness and is therefore outstandingly suitable as a substrate for the production of electronic circuits. Besides rounding the edge of the semiconductor wafer, the shaping process steps involve in particular lapping and grinding the sides. The latter two mechanical processing steps may be applied together, lapping followed by grinding, or in such a way that only one of the two processing steps is carried out. The grinding of a side may be subdivided into coarse and fine grinding steps. In coarse grinding, material is removed more coarsely and rapidly, leaving greater roughness, as well as damage extending more deeply into the crystal lattice. In fine grinding, the situation is the reverse. The grinding may furthermore be restricted to one side of the semiconductor wafer, or include both sides of the semiconductor wafer. If both sides are to be ground, then this may be done successively or in one step. The wafer geometry achieved at the end of the shaping process steps, which is a measure of the shape achieved, and which, for example is reflected by the thickness variation or overall planarity, cannot generally be further increased by subsequent processing steps which smooth the surfaces. Rather, the problem is encountered that the geometry achieved after the shaping is only worsened by subsequent processing steps.
Since superficial regions of the semiconductor wafer become damaged in the course of the shaping process steps, it is also an aim of the subsequent smoothing process steps to remove the damaged regions. These processing steps include in particular etching and polishing steps. During the etching, a liquid etchant is allowed to act on the semiconductor wafer, for example by immersing the semiconductor wafer in the etchant and optionally forcing the etchant to flow around the wafer. It is known that liquid etchants with an alkaline pH, which contain for example KOH, scarcely compromise the semiconductor wafer's geometry achieved after the shaping process steps. However, they have the disadvantage of causing an increase in the roughness of the surface. Furthermore, alkaline etchants often are a source of contamination of the semiconductor wafer by traces of metal. Liquid etchants with a pH in the acid range, for example those which contain a mixture of HF and HNO3, do not exhibit this disadvantage but perform less well than alkaline etchants as related to preserving the wafer geometry. It has therefore already been proposed to use alkaline and acidic etchants in succession, in which case their respective advantages can be combined and their respective disadvantages have less of an effect.
U.S. published application 2004/0020513 A1 describes a method for thinning a semiconductor wafer by means of gas phase etching in which HF in the gaseous state, generally as vapor, acts on the semiconductor wafer and dissolves oxide present on the semiconductor wafer to form water and SiF4. In order to form the oxide, an oxidant such as O3 is required, which is first allowed to act on the semiconductor wafer's surface to be processed with HF. The timing of the process may also be configured so that HF and the oxidant act simultaneously. Particular advantages of gas phase etching are that the etchant consumption is substantially less than when using liquid etchants, and that the cleaning of the semiconductor wafer, which is carried out after etching with a liquid etchant, does not need to be so intense, or may even be entirely eliminated.
Parameters such as freedom from defects freedom and planarity, which the producers of electronic circuits require in particular for the front surface of a semiconductor wafer, are much too demanding to be fulfilled only with the aid of etching steps. Rather, it is also necessary to carry out at least one polishing step by which damaged crystal regions are removed and the roughness of at least the front side is reduced. It is found particularly expedient to employ stock polishing, including both sides of the semiconductor wafer, with material removal of approximately at least 10 μm per wafer side, which is followed by mirror polishing restricted to the front side with material removal of less than 1 μm. The stock polishing of both sides may be carried out as simultaneously performed double-sided polishing, or may comprise two single-sided polishing steps carried out successively. Against the advantageous effect of polishing, there are the disadvantages that polishing represents a comparatively expensive processing step which consumes high-grade polishing agents and polishing cloths, and that the stock polishing unfavorably affects the semiconductor wafer's geometry achieved by the shaping process steps. Furthermore, metal traces contained in the polishing agent may be transferred onto and into the semiconductor wafer, an effect which also depends on the duration of the polishing.